In fields of laser processing, laser communication, and the like, a fiber laser device, a fiber amplifying device, and the like have been widely used. Such devices each generally include (i) an amplification fiber which is a double cladding fiber in which a core to which an active element (e.g., a rare earth ion such as ytterbium) is added and (ii) a transmission fiber which is a single cladding fiber (a single mode fiber) or a double cladding fiber.
Excitation light enters an inner cladding of the amplification fiber, via a combiner which is inserted into the amplification fiber. The excitation light is used to cause an active element, added to a core of the amplification fiber, to make a transition to a population inversion state. A fused point, where the amplification fiber and the transmission fiber are fused to each other, is usually covered with a high-refractive index resin. Residual excitation light, which remains without being absorbed by the active element, is absorbed by the high-refractive index resin.
Note, however, that residual excitation light having high energy enters a high-refractive index resin, in a case where a high-powered semiconductor laser is to be employed as an excitation light source so as to meet the demand for high power. Moreover, core light, which has high energy and is directed, at the fused point, from the core of the amplification fiber into the cladding of the transmission fiber, also enters the high-refractive index resin. In a case where the high-refractive index resin is a transparent resin, the high-refractive index resin transmits most of leakage light which has entered the high-refractive index resin. However, the leakage light which has entered the high-refractive index resin is partially absorbed by the high-refractive index resin, so that the high-refractive index resin generates heat. This accelerates a deterioration in high-refractive index resin, thereby causing a problem that reliability of the device is deteriorated.
Examples of a technique for addressing such a problem encompass an optical fiber protector disclosed in Patent Literature 1. Patent Literature 1 discloses an optical fiber protector for protecting a fusion-connecting part where double cladding fibers are fused to each other (or a fusion-connecting part of a double cladding fiber and a single cladding fiber). Such an optical fiber protector covers the fusion-connecting part with a transparent resin which is fixed to a part on a housing groove provided in a heat radiating plate.